Many spectroscopic measurement methods employing lasers have been developed for the sensitive detection of gas phase species. A common method is laser absorption, in which a laser beam is transmitted through a sample medium and the fraction of the light which is absorbed by gases contained in the sample is measured. Through knowledge of the absorption spectra of different gases, the concentration of particular gases which absorb light at the laser wavelength can be determined. Since the fraction of laser light which is absorbed is proportional to the concentration of the absorbing gas, the sensitivity for measuring small gas concentrations can be improved by removing any sources of noise or interference which otherwise restrict the minimum amount of absorption that can be measured. Laser absorption methods have many applications in laboratory research, measurement of trace species in the natural or polluted atmosphere, detection of gaseous impurities, toxic gas monitoring, in situ monitoring of combustion or other chemical processes, etc.
Optical interference fringe effects impose severe sensitivity restrictions on measurements of gas phase species using laser absorption methods. Typically, fringes, which manifest themselves as interference fringes in transmission spectra, are created by the scattering of coherent laser light from optical surfaces, such as those on mirrors, lenses, windows, and the like, disposed within the light's path. Fringes occur when light which is scattered from an optical surface is scattered again such that it reaches the optical detector and interferes with the primary transmitted beam. Fringes are especially common in systems using multiple pass cells. These cells are used to enhance the sensitivity of absorption measurements by greatly extending the path length over which absorption occurs, but they also increase the likelihood of fringe generation due to multiple reflections from mirror surfaces within the cell, thereby negating any advantage gained by increased path length.
The sensitivity of a laser absorption measurement system can be described as the minimum amount of laser absorption which can be measured. For example, with laser power levels in the range of 0.01 to 1.0 milliwatt (mW), which is typical of mid- or near-infrared diode lasers, and commercially available infrared detectors, detector noise-limited sensitivity is generally equivalent to approximately 10.sup.-6 or 10.sup.-7 fractional absorption. In multiple pass cells, the fringe amplitudes typically generated are many orders of magnitude larger than the detector noise level. For example, using a conventional "White cell" multiple pass design, Reid, et al., (J. Reid, M. El-Sherbiny, B. K. Garside, and E. A. Ballik, "Sensitivity Limits of a Tunable Diode Laser Spectrometer, with Application to the Detection of NO.sub.2 at the 100-ppt Level," Appl. Opt. 19, 3349 (1980)) have reported fringe amplitudes typically greater than 10.sup.-3 equivalent absorption. In Herriott cells, when used with single frequency diode lasers, as disclosed in Silver and Stanton, "Optical Interference Fringe Reduction in Laser Absorption Experiments" (Appl. Opt., in press), fringe amplitudes usually range from about one to five.times.10.sup.-4 equivalent absorption. Thus, the presence of the fringes seriously degrades the sensitivity, potentially by a factor of 1000 or more. In many applications, the fringes produced can render laser absorption instrumentation essentially useless. For example, in measurement of trace atmospheric species, it is important to measure many gases that are present at concentrations of less than one part-per-billion (ppb). Diode laser instrumentation, when used with an absorption path length of greater than 1 or 2 meters, as can be easily attained with standard multipass cell designs, has the potential of sub-ppb detection sensitivity if it can be operated near the detector noise limit. Therefore, the presence of interference fringes can severely restrict the application of such instrumentation by limiting detection sensitivity to concentrations near the part-per-million, rather than the desired part-per-billion level.
Fringe filtering has been conducted using both post measurement digital filtering of data and active electronic filtering as data is generated. Unfortunately, the characteristic frequency (free spectral range) of the fringes is often similar to the frequency widths of the gas absorption features under study, so that filtering techniques cannot adequately discriminate between absorption lines and interference fringes. Assume the free spectral range is c/2L.eta. where c is the speed of light, .eta. is the index of refraction of the gaseous medium in the absorption path (.eta..congruent.1), and L is the distance between the optical surfaces which give rise to the interference effects. In multiple pass cells, fringes typically arise due to light scattered from the same mirror surface on succeeding passes, so that L=2d.sub.s, where d.sub.s is the separation distance between the mirrors forming the multipass cell. For mirror separations of 0.25 to 2.5 meters, a range of experimentally convenient cell lengths for laboratory applications, the corresponding characteristic fringe spacing ranges from 300 MHz to 30 MHz. Typical infrared molecular absorption lines in the low pressure (Doppler) limit also have widths (full width at half maximum) within this range.
Minimization of interference fringe effects has been carried out using several approaches. Complex wavelength modulation methods such as disclosed in Reid, et al., "Sensitivity Limits of Tunable Diode Laser Spectrometer, with Application to the Detection of NO.sub.2 at the 100-ppt Level" (Appl. Opt. 19, 3349 (1980)); and Cassidy, et al., "Harmonic Detection with Tunable Diode Lasers - Two-Tone Modulation" (Appl. Phys. B 29, 279 (1982)) have been used. Such methods discriminate between fringes and absorption lines only when a difference exists between the characteristic widths of the fringes and the absorption lines. Furthermore, such laser modulation methods are specific to a particular laser or particular measurement and may require modification for lasers having different wavelength tuning properties or gas absorption lines having different widths.
Another approach to minimizing the effects of interference fringes is to perform successive measurements, one with the gas to be measured present and one with the gas removed, subtracting one scan from the other to obtain only the absorption spectrum of the gas. Removal of the gas may not be practical in many situations, such as on-line measurements of chemical processes, in situ measurements of atmospheric species, and the like. Even slight changes in the index of refraction of the gas mixture or the temperature of the system can shift the position of the fringes relative to the absolute laser frequency between laser wavelength scans. In addition, such approaches at least double the measurement time and require post-measurement processing of the data.
Yet another approach for minimizing interference fringe effects has been described in Webster, "Brewster-Plate Spoiler: A Nevel Method for Reducing the Amplitude of Interference Fringes that Limit Tunable-Laser Absorption," J.Opt. Soc. Am. B 2, 1464 (1985), and U.S. Pat. No. 4,684,258. Webster positions a transmissive plate in the beam path approximately at Brewster's angle between the optical surfaces which give rise to the interference fringes. Webster angularly oscillates the transmissive plate about Brewster's angle which, in effect, continuously varies the optical path length between the fringe-forming optical surfaces, thereby reducing the fringes on a time-averaged basis. The Webster device has several disadvantages and is not practical when the interference fringes are formed within a multiple pass cell. One major disadvantage is that an additional element must be introduced into the optical path. This greatly increases overall transmission losses within a system. The Brewster plate surfaces can also scatter laser light, thereby causing new fringes to be formed. This is quite likely if the plate is inserted into the path in a region where the laser beam diameter is large, that is, where the laser beam intercepts a significant fraction of the area of the plate. The plate additionally causes substantial displacement of the beam, unless the plate is very thin. Most infrared transmissive materials, such as calcium flouride and KRS-5, are readily available only in thicknesses greater than 3 mm. Plates of such thickness create beam displacements on the order of 1 mm or greater. In addition, because the index of refraction of such a plate varies as a function of wavelength, the orientation of the plate or the plate material itself will need to be changed if the laser wavelength is changed sufficiently.
The Brewster plate method of Webster is especially impractical for use in multiple pass cell systems. To reduce fringes generated by such a cell, a Brewster plate would have to be positioned inside the cell, between its multipass mirrors. In many measurement applications, such cells are vacuum tight and are operated at reduced pressure. Thus, mounting and controlling a Brewster plate within such a cell would pose design complications. For some applications, such as in situ monitoring of chemical processes, placement of any foreign element within a multipass cell may be entirely infeasible. In addition, in typical standard multipass cell designs such as "White" cells and "Herriott" cells, the various traverses of the cell by the laser beam are not confined to a small transverse area. Therefore, a Brewster plate would need to be similar in size to the multipass mirrors to ensure interception by the plate of all traverses of the cell. Scattering of laser light from such a large surface to one or more of the multipass mirrors is highly probable. Such scattering would create additional unwanted interference fringes which could not be eliminated by oscillating the plate.
Another significant disadvantage of using a Brewster plate in a multiple pass cell is that introduction of the Brewster element results in significant and unacceptable transmission losses, especially when used with randomly polarized laser beams. The transmission through such an element after n passes is T=T.sub.o.sup.n, where T.sub.o is the single pass transmission of the plate. If this single pass transmission is 80%, which is typical for infrared transmitting materials, after 40 passes, T=(0.8).sup.40 =1.3.times.10.sup.-4. Hence, only about 0.01 percent of the available light is transmitted through the cell, creating a severe loss of sensitivity in most cases. Even if 99% single pass transmission is achieved by the use of antireflection coatings, the net transmission after 40 passes is reduced by 1/3, substantially compromising sensitivity in most applications.
One object of the present invention is to eliminate interference fringes and thereby substantially increase sensitivity in laser absorption measurements.
Another object of the invention is to increase detection sensitivity in multiple pass cell laser absorption measurement systems.
One advantage of the instant invention is that in accordance therewith, sensitivity of selected laser detection systems is sufficiently enhanced to enable their use in detecting the presence of substances previously undetectable using such systems.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawing, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.